The ever-increasing traffic density has spurred pedestrians and cyclists to make themselves more visible in twilight or nightly illumination conditions in order to survive. To this end they carry retroreflective strips either as arm or leg bands or integrated into their garments or on their bikes. Such lifesaving gadgets are generally made in the form of strips. Characteristic for such retro-reflective strips is that upon illumination by a beam of light, they return the light in a cone having an axis in the direction of the impinging beam. The term retroreflective is used here to discriminate the reflection mechanism from diffusive reflection—in which the impinging light is scattered in all directions evenly—or specular reflection—when the incident light is reflected away under an equal but opposite angle to the normal on the surface.
There are a number of means available to entail this retroreflectivity into an article. Cube corner reflectors are generally used in reflectors that are rigid as for example in the fixed reflectors of a car. As such they cannot be used on flexible substrates such as strips or rope like material. In order to overcome this latter difficulty, very small (smaller then 1 mm) glass pyramids have become available. When such glass pyramids are embedded into a coating on a substrate some of the pyramids will be properly aligned with the incident beam and effectively shine back the light. Such a structure can be recognised in the dark because it tends to twinkle when one is moving by, the twinkling being due to different micro pyramids that become properly aligned with the impinging beam. The micro pyramids are less preferred for products with a limited area available for retroreflection (such as e.g. spokes of a bicycle wheel). By far the most preferred retroreflective means are very small ‘glass beads’ made out of a highly refracting, transparent material. Indeed, geometrical optics readily shows that a caustic focal region of a transparent sphere forms close to the surface opposite from the impinging light on the optical axis. When now a reflective layer is provided on the back surface, most of the converging light will return in a cone around the incident beam direction. It is readily shown that the higher the refractive index of the material is, the smaller the focus region becomes while it nears the sphere and eventually enters the sphere (for refractive indices larger than 2). Likewise, it can be shown that the cone angle of the returning beam becomes smallest for materials with a refractive index of about 1.547, while smaller and larger refractive indices show larger exit cones. Nowadays, tiny beads with a size between 10 and 500 microns are readily available.
Such glass beads are used in coatings on predominantly planar structures such as strips, or traffic signs or the like. With planar structures of a given area a lot of light is readily caught and returned, while elongated, cylindrical structures having the same retroreflective area do not have a large head-on reflection as already half of the area is concealed behind the carrier. However, elongated cylindrical structures have the advantage that they keep on reflecting for each radial incident beam of light while for a planar structure the reflection readily diminishes when the beam strikes the surface off normal.
Most of the methods known today to make an elongated retroreflecting product rely on the use of strips that on themselves are retroreflective. Known techniques to apply those strips on elongated products are taping and wrapping. An example of this is given in U.S. Pat. No. 6,355,349 wherein a steel wire rope is wrapped into a reflective strip and subsequently coated with a clear coating. Another way to make retroreflective elongated products is to start from a retroreflective foil that is slit into narrow strips. These ‘threads’ are twined together into a yarn or rope as described in U.S. Pat. No. 4,697,407. US 2004/0180199 is a publication that describes a method to embed the retroreflective beads or pyramids into a polymer that is subsequently melt spun to form a yarn. So here the beads are inside the thread and not only at the surface.
Working with strips that are wrapped or taped around elongated products entails some drawbacks. As these products are normally used outdoors, there is a risk that the strip loosens from the product even if an adhesive has been used, due to the weather conditions. There is also a cost disadvantage in that first a retroreflective tape has to be made that is then subsequently wrapped around the product. There is also a tendency for the wrapped strip to tear when the elongated product is bent.